The invention relates to an attachment buckle for fitting and adjusting a strap, composed of a pair of superposed rings presenting rectangular frames of different dimensions, the larger first ring comprising a rectangular first opening arranged under a rectangular second opening arranged in the second ring of smaller size so as to define a transverse slot for the strap to pass through, the slot being arranged between one of the outside edges of the second ring and the adjacent side of the first opening of the first ring, the buckle being either in a position blocking the strap in the slot following application of a clamping force, or in an releasing position after a swivelling movement of the second ring resulting in the passage zone in the slot being enlarged.